1. Field of the Invention
The present invention relates to electrostatic propulsion engines and, in particular, to electric propulsion thrusters used in spacecraft.
2. Description of the Prior Art
Ion propulsion and arcjet systems have been proposed and tested for many years to provide propulsion and control in the near vacuum environment of space by use of the simple action-reaction result of thrusting material out in one direction to cause motion in the other direction. Although various propulsion systems have achieved success for various types of missions, there is a gap in propulsion capability for certain missions such as orbit transfer missions, and North-South station keeping for geosynchronous satellites, where relatively high propellant exhaust velocities (relative to chemical propulsion) are required at higher propulsion system efficiencies than are available with conventional ion engines or arcjets.
Conventional ion propulsion engines use ions of relatively heavy elements, such as xenon, because thruster efficiency is known to increase with the use of heavy ions. In addition, heavier propellants allow for the development of thrusters which can operate at high thrust densities in combination with high power levels which would be prohibitive for lighter propellants. Cluster ion propulsion, using ionized clusters bound primarily by van der Waals forces, has been investigated since the 1960's in attempts to address the thruster efficiency performance problems at moderate exhaust velocities by providing propellants heavier than would otherwise be available. The theoretical advantages of using a clustered propellant arise from the decreased cost of ion production for the same mass flux through a thruster that could be achieved with a monatomic propellant.
However, satisfactory propellants for use in such systems must have a relatively narrow, preferably monodisperse, mass distribution and a high resistance to fragmentation to minimize efficiency loss. In addition, large quantities of the propellant must conveniently be made available on board the spacecraft when required for use. Convenient propellants providing high thruster efficiencies and narrow charge-to-mass ratios suitable for use in ion propulsion systems have not yet been suitably identified.
The conventional approach to cluster ion propulsion systems utilized various methods for producing and ionizing such clusters in the propulsion system itself. These methods include condensing clusters from a supersaturated gas stream in supersonic expansion followed by subsequent ionization by either electron bombardment or electron attachment.
Ion nucleation, in which some of the atoms in the gas flow are ionized to serve as nucleation centers, was also studied and proposed as a way to improve ion-engine performance. This approach does improve ion production efficiency and eliminates the problems of cluster fragmentation. However, the method produces a broad range of cluster sizes so that theoretical advantages of cluster ion propulsion are not fully realized in practice.
What is needed is a convenient and inexpensive technique for producing and storing preformed heavy ion propellant with near monochromatic mass distribution and reduced tendency to fragment which can be used to provide increased thruster efficiency at moderate exhaust velocities in ion propulsion systems.